I really appreciate all the feedback — these online resources are major saviours.
Idolse — Calibre doesn't cause the hyphenation problem — that's an iBooks thing (iBooks does hyphenation automatically and really badly). The earlier version of Calibre did something to the ePub to remove this auto-hyphenation. The latest version of Calibre seems not to do this any more (trying the various suggestions posted here and combinations thereof). When I get some time I will definitely get a couple of files together and log a bug report for you.
I've since found (thanks to
Liz Castro via Rick Gordon) that adding "-webkit-hyphens:none;" in the CSS will achieve exactly what I need — no auto-hyphenation in iBooks.
d.